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1. Extreme Ultraviolet Reflection-Absorption (XUV-RA) Spectroscopy: Probing Dynamics at Surfaces from a Molecular Perspective

   https://doi.org/https://doi.org/10.1021/acs.accounts.1c00765  

   Biswas, Somnath ; Baker, L. Robert ( March 2022 , Accounts of chemical research)

   Extreme ultraviolet (XUV) light sources based on high harmonic generation are enabling the development of novel spectroscopic methods to help advance the frontiers of ultrafast science and technology. In this account we discuss the development of XUV-RA spectroscopy at near grazing incident reflection geometry and highlight recent applications of this method to study ultrafast electron dynamics at surfaces. Measuring core-to-valence transitions with broadband, femtosecond pulses of XUV light extends the benefits of x-ray absorption spectroscopy to a laboratory tabletop by providing a chemical fingerprint of materials, including the ability to resolve individual elements with sensitivity to oxidation state, spin state,more »carrier polarity, and coordination geometry. Combining this chemical state sensitivity with femtosecond time resolution provides new insight into the material properties that govern charge carrier dynamics in complex materials. It is well known that surface dynamics differ significantly from equivalent processes in bulk materials, and that charge separation, trapping, transport, and recombination occurring uniquely at surfaces governs the efficiency of numerous technologically relevant processes spanning photocatalysis, photovoltaics, and information storage and processing. Importantly, XUV-RA spectroscopy at near grazing angle is also surface sensitive with a probe depth of 3 nm, providing a new window into electronic and structural dynamics at surfaces and interfaces. Here we highlight the unique capabilities and recent applications of XUVRA spectroscopy to study photo-induced surface dynamics in metal oxide semiconductors, including photocatalytic oxides (Fe2O3, Co3O4 NiO, and CuFeO2) as well as photoswitchable magnetic oxide (CoFe2O4). We first compare the ultrafast electron self-trapping rates via small polaron formation at the surface and bulk of Fe2O3 where we note that the energetics and kinetics of this process differ significantly at the surface. Additionally, we demonstrate the ability to systematically tune this kinetics by molecular functionalization, thereby, providing a route to control carrier transport at surfaces. We also measure the spectral signatures of charge transfer excitons with site specific localization of both electrons and holes in a series of transition metal oxide semiconductors (Fe2O3, NiO, Co3O4). The presence of valence band holes probed at the oxygen L1-edge confirms a direct relationship between the metal-oxygen bond covalency and water oxidation efficiency. For a mixed metal oxide CuFeO2 in the layered delafossite structure, XUV-RA reveals that the sub-picosecond hole thermalization from O 2p to Cu 3d states of CuFeO2 leads to the spatial separation of electrons and holes, resulting in exceptional photocatalytic performance for H2 evolution and CO2 reduction of this material. Finally, we provide an example to show the ability of XUV-RA to probe spin state specific dynamics in a the photo-switchable ferrimagnet, cobalt ferrite (CoFe2O4). This study provides a detailed understating of ultrafast spin switching in a complex magnetic material with site-specific resolution. In summary, the applications of XUV-RA spectroscopy demonstrated here illustrate the current abilities and future promise of this method to extend molecule-level understanding from well-defined photochemical complexes to complex materials so that charge and spin dynamics at surfaces can be tuned with the precision of molecular photochemistry.« less
2. A localized view on molecular dissociation via electron-ion partial covariance

   https://doi.org/10.1038/s42004-022-00656-w  

   Allum, Felix ; Music, Valerija ; Inhester, Ludger ; Boll, Rebecca ; Erk, Benjamin ; Schmidt, Philipp ; Baumann, Thomas M. ; Brenner, Günter ; Burt, Michael ; Demekhin, Philipp V. ; et al ( December 2022 , Communications Chemistry)

   Abstract Inner-shell photoelectron spectroscopy provides an element-specific probe of molecular structure, as core-electron binding energies are sensitive to the chemical environment. Short-wavelength femtosecond light sources, such as Free-Electron Lasers (FELs), even enable time-resolved site-specific investigations of molecular photochemistry. Here, we study the ultraviolet photodissociation of the prototypical chiral molecule 1-iodo-2-methylbutane, probed by extreme-ultraviolet (XUV) pulses from the Free-electron LASer in Hamburg (FLASH) through the ultrafast evolution of the iodine 4d binding energy. Methodologically, we employ electron-ion partial covariance imaging as a technique to isolate otherwise elusive features in a two-dimensional photoelectron spectrum arising from different photofragmentation pathways. The experimental andmore »theoretical results for the time-resolved electron spectra of the 4d 3/2 and 4d 5/2 atomic and molecular levels that are disentangled by this method provide a key step towards studying structural and chemical changes from a specific spectator site.« less
3. Ultrafast dynamics of exciton formation and decay in two-dimensional tungsten disulfide (2D-WS ₂ ) monolayers

   https://doi.org/10.1039/d0cp03220d  

   Eroglu, Zeynep Ezgi ; Comegys, Olivia ; Quintanar, Leo S. ; Azam, Nurul ; Elafandi, Salah ; Mahjouri-Samani, Masoud ; Boulesbaa, Abdelaziz ( August 2020 , Physical Chemistry Chemical Physics)

   Excitons in two-dimensional transition metal dichalcogenide monolayers (2D-TMDs) are of essential importance due to their key involvement in 2D-TMD-based applications. For instance, exciton dissociation and exciton radiative recombination are indispensible processes in photovoltaic and light-emitting devices, respectively. These two processes depend drastically on the photogeneration efficiency and lifetime of excitons. Here, we incorporate femtosecond pump–probe spectroscopy to investigate the ultrafast dynamics of exciton formation and decay in a single crystal of monolayer 2D tungsten disulfide (WS 2 ). Investigation of the formation dynamics of the lowest exciton (X A ) indicated that the formation time linearly increases from ∼150 fsmore »upon resonant excitation, to ∼500 fs following excitation that is ∼1.1 eV above the band-gap. This dependence is attributed to the time it takes highly excited electrons in the conduction band (CB) to relax to the CB minimum (CBM) and contribute to the formation of X A . This is confirmed by infrared measurements of electron intraband relaxation dynamics. Furthermore, pump–probe experiments suggested that the X A ground state depletion recovery dynamics depend on the excitation energy as well. The average recovery time increased from ∼10 ps in the case of resonant excitation to ∼50 ps following excitation well above the band-gap. Having the ability to control whether generating short-lived or long-lived electron–hole pairs in 2D-TMD monolayers opens a new horizon for the application of these materials. For instance, long-lived electron–hole pairs are appropriate for photovoltaic devices, but short-lived excitons are more beneficial for lasers with ultrashort pulses.« less
4. Unveiling the spatial distribution of molecular coherences at conical intersections by covariance X-ray diffraction signals

   https://doi.org/10.1073/pnas.2105046118  

   Cavaletto, Stefano M. ; Keefer, Daniel ; Rouxel, Jérémy R. ; Aleotti, Flavia ; Segatta, Francesco ; Garavelli, Marco ; Mukamel, Shaul ( May 2021 , Proceedings of the National Academy of Sciences)

   The outcomes and timescales of molecular nonadiabatic dynamics are decisively impacted by the quantum coherences generated at localized molecular regions. In time-resolved X-ray diffraction imaging, these coherences create distinct signatures via inelastic photon scattering, but they are buried under much stronger background elastic features. Here, we exploit the rich dynamical information encoded in the inelastic patterns, which we reveal by frequency-dispersed covariance ultrafast powder X-ray diffraction of stochastic X-ray free-electron laser pulses. This is demonstrated for the photoisomerization of azobenzene involving the passage through a conical intersection, where the nuclear wave packet branches and explores different quantum pathways. Snapshots ofmore »the coherence dynamics are obtained at high frequency shifts, not accessible with conventional diffraction measurements. These provide access to the timing and to the confined spatial distribution of the valence electrons directly involved in the conical intersection passage. This study can be extended to full three-dimensional imaging of conical intersections with ultrafast X-ray and electron diffraction.

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5. Observation of Ultrafast Coherence Transfer and Degenerate States with Polarization-Controlled Two-Dimensional Electronic Spectroscopy

   https://doi.org/10.1021/acs.jpcb.0c08126  

   Sardjan, Andy S. ; Westerman, Floris P. ; Ogilvie, Jennifer P. ; Jansen, Thomas L. ( September 2020 , The Journal of Physical Chemistry B)

   Optical spectroscopy is a powerful tool to inter- 7 rogate quantum states of matter. We present simulation results for 8 the cross-polarized two-dimensional electronic spectra of the light- 9 harvesting system LH2 of purple bacteria. We identify a spectral feature on the diagonal, which we assign to ultrafast coherence transfer between degenerate states. The implication for the interpretation of previous experiments on different systems and the potential use of this feature are discussed. In particular, we foresee that this kind of feature will be useful for identifying mixed degenerate states and for identifying the origin of symmetry breaking disorder inmore »systems like LH2. Furthermore, this may help identify both vibrational and electronic states in biological systems such as proteins and solid-state materials such as hybrid perovskites.« less